The 65-year-old filmmaker is sitting in his East Cambridge offices, a cleanly designed floor-length warren of cubicles, editing bays, bookshelves, and objets d’art. His personal office is shaded against the afternoon sun, a cool oasis of eccentricity. A stuffed baby penguin stands on a side table and a horse’s head protrudes from the wall. On his desk, in a glass case, is a monkey’s head — a Hanukkah present from his wife. The office, like the mind of the man who presides over it, is a cabinet of curiosities.

That mind is more restless than it has ever been. Morris is a Boston institution and a national treasure for the nine documentaries he has made during the past 30 years. 1988’s “The Thin Blue Line” freed an innocent man from prison. 2003’s “The Fog of War,” in which Robert McNamara broods over his part in the Vietnam War, won an Oscar. And 2008’s “Standard Operating Procedure” probed the nature of photography and the sins of Abu Ghraib. “Gates of Heaven” (1978) remains the greatest film about pet cemeteries ever made.

Yet at the moment Morris is busier than he has ever been — active not just in film but on all fronts. He has published two books in the last three years; the most recent, “A Wilderness of Error,” opens up the 1970 Jeffrey MacDonald murder case for reappraisal. He is putting the finishing touches on “The Unknown Known,” a documentary on former Secretary of Defense Donald Rumsfeld, to meet the fall film festival season, after which it should go into theatrical release. “Inevitably, there will be comparisons to ‘The Fog of War,’ ” the director says, “but it’s a very different kind of story. It’s a story about someone absolutely convinced of their own rectitude.”

His essays on history, imagery, evidence, and the knowability of facts spill onto the New York Times website and Slate.com. He’s an active, if not obsessive, tweeter (@errolmorris). Morris has also signed up to direct not one, but two fictional feature films; the one about cryonics, “Freezing People is Easy,” is set to star Paul Rudd and Owen Wilson, while “Holland, Michigan” is a “Hitchcock-like movie” set at a tulip festival. And his second career making commercials — he has shot more than 1,000 spots for everything from Target to Cisco — would be enough of a first career for many people.

For all that, the most important thing Morris may do this year is put his name on another man’s movie. “The Act of Killing,” a unique documentary in which the squad leaders of Indonesia’s mid-1960s mass killings confront their crimes by reenacting them for the camera, is the work of the young Harvard-educated filmmaker Joshua Oppenheimer. Credited as “executive producers,” though — and very much responsible for the film getting funded and seen — are Morris and his own onetime mentor, Werner Herzog.

“I think it’s a great movie,” Morris says of “The Act of Killing,” which opens in the Boston area on Friday. “Because of its strangeness, its ambitions, because in many ways it extends what I think a documentary film could be. I’ve never thought of doing anything like this. Of getting the perpetrators of a crime to reenact it? I mean, what a crazy idea. Can you imagine a movie with Whitey Bulger and Steve ‘The Rifleman’ Flemmi reenacting their crimes?” Not surprisingly, Oppenheimer’s film has stirred controversy, with some accusing him of collaborating with war criminals and others — like Morris — rushing to his defense.

Decades ago, when Morris was an unknown director struggling to make “Gates of Heaven,” Herzog promised to cook and eat his shoe if that film was ever completed. He made good on the bet, an event duly captured in Les Blank’s deadpan 1980 documentary “Werner Herzog Eats His Shoe.” Oppenheimer is aware of the history and grateful to be the latest in this daisy-chain of established filmmakers lending a hand (and a foot) to the new guys.

“There’s a really beautiful kind of closing of the circle for me,” Oppenheimer says in a phone interview, “because Werner was supportive of Errol’s work back when he ate his shoe to help promote ‘Gates of Heaven.’ So when I realized they both were enamored of ‘The Act of Killing’ and were working to support it in the way they are, it really felt like this meaningful embrace from two true inspirations.”

Morris’s backing speaks to his many aspects, certainly his career-long fascination with evidence and the narratives (not all of them true) we decant from it. But Morris’s mentorship of Oppenheimer — not to mention the influential late film editor Karen Schmeer and the many local film students who pass through jobs in his Fourth Floor Productions offices — is an indication of the stature he now commands in the worlds of independent movies, documentary filmmaking, the Boston production scene, and a larger national arena of ideas. Like Michael Moore and Ken Burns, Errol Morris is a brand name of the documentary renaissance, but his brand connotes a very particular and quixotic investigative sensibility — one that has widened the genre and arguably made an audience for a film like Oppenheimer’s possible.

“To me, ‘investigative’ means something specific,” says Morris. “It means you don’t know where you’re going when you start out. ‘Thin Blue Line’ is a good example. ‘The Act of Killing’ is a good example. Where you’re drawn into a story and it changes as you go along, sometimes radically.”

He could be talking about the scene in 2010’s “Tabloid” where we see Joyce McKinney’s nude photos and her carefully manufactured self-image as a put-upon Southern belle begins to crumble. Or about the climactic scene in Oppenheimer’s film in which a complacent aging assassin named Anwar Congo looks at his own reenactment footage and starts dry-heaving uncontrollably.

Alfred Guzzetti, a filmmaker, professor, and former head of Harvard’s Visual and Environmental Studies program, says his students see Morris “as a major figure, because he was very influential in documentaries coming back into theaters.” Guzzetti befriended the director when Morris relocated from New York to Boston 25 years ago, and the two men still meet weekly to play chamber music together, Guzzetti on piano, Morris on cello.

“He’s patented a certain style,” Guzzetti says, “and that style is widely imitated: the combination of these very ironic interludes and comparisons, the way in which he edits sync-sound talking heads, the driving minimalist music — I don’t know whether he agrees with this, but I’ve always told him that I think that the films are kind of oratorios.”

Morris lives near Inman Square in Cambridge, with his wife, Julia Sheehan, an art historian and co-producer of his films whose onetime job at the Boston Children’s Museum was responsible for the couple relocating to New England from New York City in 1988. (Their son, Hamilton Morris, 26, is the science editor of Vice magazine in Manhattan.)

While “Tabloid” was edited in California, the director prefers to work closer to home, using production facilities in Allston and editing “The Unknown Known” at his Cambridge offices. Says Morris, simply, “I think I would have gone down in L.A. Part of my survival is because I am here, in the sense that I’m removed from all that.”

He isn’t a constant figure on the Boston production or academic scenes, nor is he photographed at local premieres. Teacher-documentarians like Ross McElwee and Robb Moss at Harvard, Mary Jane Doherty at BU, Gerald Peary at Suffolk, and Jane Gillooly at the MFA are more visible presences. Morris, by contrast, is the maestro on the hill — an Oz-like figure whose influence ripples out to the horizons and who increasingly, if somewhat shyly, is venturing out from behind his curtain.

His newfound career as an author is one such area. “Writing is never for me an attempt to make money,” he insists. “It’s usually an attempt to lose money, and I’m usually successful. I just think things should be about thinking things through.”

Morris’s very active TV commercial business also helps him stay in Boston while keeping him apart from local colleagues. Though his peak of productivity might have been the Miller High Life “High Life Man” campaign of 1998 through 2005 — more than 100 TV spots done in a whimsical faux-macho style, they can be viewed at Morris’s website (errolmorris.com/commercials.html) — he still logs 30 to 40 commercial shoot days a year.

Lyda Kuth, a filmmaker and executive director of the documentary support organization the LEF Foundation, says “Errol really figured how to make his work by having a very vibrant commercial business to support himself. I don’t think I know anyone else in the Boston community who’s been able to do that.”

Nor anyone whose work has so broadened the acceptance of documentaries as a robust commercial form. “It’s quite amazing,” Morris says. “People are making documentaries that go into theaters routinely. Twenty, 30 years ago, that was just unheard of.”

He acknowledges that “I’m part of that,” but he’s more excited to talk about “The Act of Killing” and the moral conundrum it forces upon its subjects and its audience.

“Here’s another thing about why what Josh has done is good. The role of a good documentary is not to convince you about what happened, but to force you to think about what happened. And if it does that, then it really has done its job.”

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